Memory cells containing metal oxides

ABSTRACT

Some embodiments include memory cells which have first and second metal oxides between first and second electrodes. The first and second electrodes include metal. The first metal oxide has at least two regions which differ in oxygen concentration relative to one another. One of the regions is a first region and another is a second region. The first region is closer to the first electrode than the second region and has a greater oxygen concentration than the second region. The first metal oxide includes one or both of hafnium oxide and zirconium oxide. The second metal oxide is directly against the first metal oxide and includes a different metal than the first metal oxide. There is a substantially linear continuous oxygen-concentration gradient extending across an entirety of the first metal oxide.

RELATED PATENT DATA

This patent resulted from a continuation of U.S. patent application Ser.No. 14/053,847, which was filed Oct. 15, 2013, which issued as U.S. Pat.No. 8,962,387, and which is hereby incorporated herein by reference;which resulted from a divisional of U.S. patent application Ser. No.13/355,382, which was filed Jan. 20, 2012, which issued as U.S. Pat. No.8,581,224, and which is hereby incorporated herein by reference.

TECHNICAL FIELD

Memory cells and methods of forming memory cells.

BACKGROUND

Memory is one type of integrated circuitry, and is used in computersystems for storing data. Integrated memory is usually fabricated in oneor more arrays of individual memory cells. The memory cells may bevolatile, semi-volatile, or nonvolatile. Nonvolatile memory cells canstore data for extended periods of time, and in some instances can storedata in the absence of power. Volatile memory dissipates and istherefore refreshed/rewritten to maintain data storage.

The memory cells are configured to retain or store memory in at leasttwo different selectable states. In a binary system, the states areconsidered as either a “0” or a “1”. In other systems, at least someindividual memory cells may be configured to store more than two levelsor states of information.

There is a continuing effort to produce smaller and denser integratedcircuits. The smallest and simplest memory cell will likely be comprisedof two electrically conductive electrodes having a programmable materialreceived between them. Such memory cells may be referred to ascross-point memory cells.

Programmable materials suitable for utilization in cross-point memorywill have two or more selectable and electrically differentiable memorystates. The multiple selectable memory states can enable storing ofinformation by an individual memory cell. The reading of the cellcomprises determination of which of the memory states the programmablematerial is in, and the writing of information to the cell comprisesplacing the programmable material in a predetermined memory state. Someprogrammable materials retain a memory state in the absence of refresh,and thus may be incorporated into nonvolatile memory cells.

Significant interest is presently being directed toward programmablematerials that utilize ions as mobile charge carriers. The programmablematerials may be converted from one memory state to another by movingthe mobile charge carriers therein to alter a distribution of chargedensity within the programmable materials. Memory devices that utilizemigration of mobile charge carriers to transition from one memory stateto another are sometimes referred to as Resistive Random Access Memory(RRAM) cells. Example RRAM cells are memristors, which may utilize anoxide (for instance, titanium oxide) as a programmable material, andwhich may utilize oxygen migration within such programmable material asa mechanism for transitioning from one memory state to another.

There can be difficulties associated with the formation of memristorsand other RRAM cells. Accordingly, it would be desirable to develop newmethods of forming memristors and RRAM cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates a process stage of an exampleembodiment method of forming a memory cell.

FIG. 2 diagrammatically illustrates an example embodiment memory cell.

FIG. 3 graphically illustrates an oxygen concentration gradient withinthe FIG. 2 memory cell.

FIGS. 4-6 graphically illustrate other oxygen gradients that may beutilized in other memory cell embodiments.

FIG. 7 diagrammatically illustrates another example embodiment memorycell.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Some embodiments include new methods of forming memristor or other RRAMcells, and some embodiments include new memory cell architectures.Example embodiments are described with reference to FIGS. 1-7.

Referring to FIG. 1, a structure 10 is diagrammatically illustrated incross-sectional side view, and is shown subjected to a treatment(represented by the arrow 15) which converts the structure 10 into amemory cell 30.

The structure 10 comprises an electrode material 12, a metal oxidematerial 14 over the electrode material, an oxygen-sink material 16 overthe metal oxide material, and another electrode material 18 over theoxygen-sink material.

The electrode materials 12 and 18 may be referred to as first and secondelectrode materials, respectively, to distinguish such electrodematerials from one another. The electrode materials 12 and 18 maycomprise the same composition as one another, or different compositions.The electrode materials 12 and 18 may comprise any suitable electricallyconductive compositions or combinations of compositions. In someembodiments, one or both of the electrode materials may comprise,consist essentially of, or consist of a noble metal; such as, forexample, platinum or palladium. In some embodiments, one or both of theelectrode materials may comprise copper. In such embodiments, the coppermay be surrounded by appropriate copper barrier material (for instance,a ruthenium-containing material, Ta, TaN, TiN, etc.) to alleviate orprevent copper migration.

The electrode materials 12 and 18 may be electrically coupled withaccess/sense lines (e.g., wordlines and bit lines). For instance, theelectrode material 12 may be part of a first access/sense line thatextends into and out of the page relative to the FIG. 1 view ofstructure 10, and the electrode material 18 may be part of a secondaccess/sense line that extends substantially orthogonally to the firstaccess/sense line. Accordingly, the metal oxide 14 may be at a regionwhere the first and second access/sense lines overlap, and thus may beincorporated into a cross-point memory cell in some embodiments.

In the shown embodiment, metal oxide material 14 is directly againstelectrode material 12, oxygen-sink material 16 is directly against metaloxide material 14, and electrode material 18 is directly againstoxygen-sink material 16. In other embodiments, one or more othermaterials may be incorporated into the memory cell so that one or moreof the illustrated direct-contact relationships is altered. Forinstance, in some embodiments electrode material 12 may be a noblemetal, and another material (for instance, a metal silicide or a metalnitride) may be provided between the metal oxide 14 and the electrodematerial 12 to improve adherence between the metal oxide and the noblemetal.

The metal oxide material 14 may comprise any suitable composition orcombination of compositions; and in some embodiments may comprise,consist essentially of, or consist of a composition selected from thegroup consisting of aluminum oxide, tantalum oxide, titanium oxide,nickel oxide, hafnium oxide and zirconium oxide.

The oxygen-sink material may comprise any suitable composition orcombination of compositions; and in some embodiments may comprise,consist essentially of, or consist of a metal selected from the groupconsisting of ruthenium, nickel, iridium, titanium and tantalum.

The materials 12, 14, 16 and 18 may be formed with any suitableprocessing, including, for example, one or more of atomic layerdeposition (ALD), chemical vapor deposition (CVD) and physical vapordeposition (PVD).

The conversion from structure 10 to memory cell 30 comprisestransferring oxygen from metal oxide 14 into the oxygen-sink material16. The transfer transforms oxygen-sink material 16 into an oxide 20,and forms an oxygen-depleted region 22 within the metal oxide 14.

In some embodiments, oxygen-sink material 16 comprises, consistsessentially of, or consists of metal; and thus oxide 20 may comprise,consist essentially of, or consist of metal oxide. In such embodiments,the metal oxides 14 and 20 of memory cell 30 may be referred to as firstand second metal oxides, respectively, to distinguish such metal oxidesfrom one another. In some embodiments, metal oxide 20 may comprise,consist essentially of, or consist of ruthenium oxide, iridium oxide,nickel oxide, tantalum oxide or titanium oxide. In the shown embodiment,and entirety of the oxygen-sink material 16 of structure 10 is convertedto oxide 20 during formation of memory cell 30. Other embodiments mayhave less than an entirety of the oxygen-sink material converted tooxide.

The formation of the oxygen-depleted region 22 subdivides the metaloxide 14 into two regions 22 and 24. A dashed line 23 is provided todiagrammatically illustrate an approximate boundary, or interface,between such regions. The region 24 retains the initial stoichiometry ofthe metal oxide, while the region 22 has a lower concentration of oxygendue to oxygen being transferred from region 22 into the oxygen-sinkmaterial 16 to form oxide 20. Although the metal oxide is subdividedinto two regions in the shown embodiment, in other embodiments the metaloxide may be subdivided into more than two regions and/or there may be agradual change in oxygen concentration rather than the illustratedabrupt interface.

FIG. 2 shows another view of the example embodiment memory cell 30. Themetal oxide material 14 may have an overall thickness (shown as 32)within a range of from about 2 nanometers (nm) to about 10 nm; the metaloxide material 20 may have a thickness (shown as 34) of less than orequal to about 4 nm (for instance, a thickness within a range of fromabout 1 nm to about 4 nm); and the oxygen-depleted region 22 of themetal oxide 14 may have a thickness (shown as 36) of less than or equalto about 3 nm, such as a thickness within a range of from about 0.5 nmto about 3 nm.

In some embodiments, the thickness of the oxygen-depleted region 22 maybe determined by the initial thickness of the oxygen-sink material 16 ofthe structure 10 of FIG. 1. Specifically, an entirety of the oxygen-sinkmaterial may be converted to metal oxide 20. Thus, the amount of oxygenconsumed by the oxygen-sink material 16 is dictated by the initialthickness of the oxygen-sink material; or in other words, the depletionregion 22 is formed in a self-limiting process (with such process beinglimited by the initial thickness of material 16). A difficulty in priorart processes of forming memristor cells occurs in attempting touniformly tailor the relative thickness of an oxygen-depleted region ofmetal oxide to a non-oxygen-depleted region of the metal oxide acrossmultiple memory cells of a memory array. Utilization of the oxygen-sinkmaterial 16 to form depletion region 22 may overcome such prior artdifficulty by linking the thickness of the depletion region to theinitial thickness of the oxygen-sink material. Thus, some embodimentstake advantage of the relative simplicity of depositing the oxygen-sinkmaterial to a desired thickness which is uniform across numerousstructures of an array, as opposed to trying to directly deposit anoxygen-depleted region of an oxide.

The oxygen-sink material 16 may be formed to be quite thin. Forinstance, in some embodiments the material 16 may have a thickness offrom about 0.5 nm to about 4 nm; and in some embodiments may have athickness of about one atomic layer.

The conversion from structure 10 of FIG. 1 to memory cell 30 may be athermodynamically-favored process such that the treatment 15 of FIG. 1is substantially irreversible (or even entirely irreversible), at leastrelative to subsequent conditions that memory cell 30 is exposed toduring the intended use of the memory cell.

The treatment 15 of FIG. 1 may comprise any suitable treatment. In someembodiments, the treatment may comprise a thermal treatment; such as,for example, a treatment in which the metal oxide material 14 andoxygen-sink material 16 are subjected to a temperature of at least about200° C. (for instance, a temperature of from about 200° C. to about 500°C.). In some embodiments, the treatment may comprise an electricaltreatment alternatively to, or in addition to, the thermal treatment.The electrical treatment may comprise flow of electrical current acrossmetal oxide material 14 and oxygen-sink material 16 to ultimatelytransfer oxygen from the metal oxide material 14 into the oxygen-sinkmaterial and thereby form the memory cell 30. In some embodiments, theelectrical treatment may form a filament (discussed below with referenceto FIG. 7) in addition to forming the oxygen-depleted region 22.

The treatment 15 utilized to form the oxygen-depleted region 22 may beconducted after formation of the electrode material 18 (as shown inFIG. 1) or prior to formation of such electrode material.

The metal oxide 20 of memory cell 30 may be electrically conductive insome embodiments (for instance, may comprise ruthenium oxide), and maybe electrically insulative in other embodiments (for instance, maycomprise titanium oxide). In some embodiments, it can be advantageousthat the metal oxide 20 be electrically conductive. In such embodimentsthe primary consideration relative to the thickness of the initial metal16 (i.e., the oxygen-sink material of structure 10 in FIG. 1) may berelated to the desired thickness of the depletion region 22. Incontrast, if the metal oxide 20 is electrically insulative, aconsideration relative to the ultimate thickness of the metal oxide 20may be that such metal oxide should be kept very thin so that it doesnot interfere with performance of memory cell 30. Thus, if oxide 20 iselectrically insulative, the thickness of the initial metal 16 may bedetermined by two considerations; with one being the desired thicknessof depletion region 22, and another being a desire to keep theelectrically insulative metal oxide 20 very thin. In contrast, if oxide20 is electrically conductive, the thickness of the initial metal 16 maybe determined by only the one consideration of the desired thickness ofdepletion region 22.

The memory cell 30 of FIG. 2 has labels 50 and 52 at the interfacesbetween materials 12, 14 and 20. Such labels are utilized in FIGS. 3-6to describe example oxygen concentration gradients that may be formed inmetal oxide material 14 in various example embodiments.

FIG. 3 graphically illustrates an oxygen concentration gradient (withthe oxygen concentration being shown along the y-axis as [O]) within thematerial 14 of FIG. 2. Specifically, the oxygen concentration isrelatively high in region 24 of metal oxide material 14, and relativelylow within region 22 of the metal oxide material 14. A step occurs inthe oxygen-concentration gradient across the interface 23 were region 22meets region 24. In the illustrated embodiment, the step is abrupt. Inother embodiments the step may be more gradual so that there is a taperin the oxygen-concentration gradient along interface 23, rather than theabrupt step.

FIG. 4 illustrates an embodiment in which there is a relatively highoxygen concentration in a domain of metal oxide material 14 adjacentinterface 50, a relatively low oxygen concentration in a domain of metaloxide material 14 adjacent interface 52, and a substantially linear,decreasing, continuous oxygen-concentration gradient extending across anentirety of the metal oxide material 14 from the interface 50 to theinterface 52.

FIG. 5 illustrates an embodiment in which there is a relatively highoxygen concentration in a domain of metal oxide material 14 adjacentinterface 50, a relatively low oxygen concentration in a domain of metaloxide material 14 adjacent interface 52, a flat oxygen-concentrationgradient across region 22 of metal oxide 14, and a substantially linear,decreasing, continuous oxygen-concentration gradient extending acrossregion 24 of the metal oxide material 14 from the interface 50 to theinterface 23.

FIG. 6 illustrates an embodiment in which there is a relatively highoxygen concentration in a domain of metal oxide material 14 adjacentinterface 50, a relatively low oxygen concentration in a domain of metaloxide material 14 adjacent interface 52, a flat oxygen-concentrationgradient across region 24 of metal oxide 14, and a substantially linear,decreasing, continuous oxygen-concentration gradient extending acrossregion 22 of the metal oxide material 14 from the interface 23 to theinterface 52.

In discussing the formation of memory cell 30 with reference to FIG. 1above, it was indicated that some embodiments may include formation of aconductive filament within metal oxide material 14 during formation ofthe memory cell. FIG. 7 shows an example embodiment memory cell 30 acomprising an electrically conductive filament 60 extending partiallythrough metal oxide material 14. Such conductive filament may be formedby providing electrical current between electrodes 12 and 18. Theconductive filament may comprise any suitable electrically conductivematerial, including, for example, electrode material 12 transported bythe flow of the electrical current. Filaments analogous to filament 60are known in the art for utilization in memristor cells, and it is alsoknown in the art to electrically form such filaments during fabricationof memristor cells.

The filament 60 extends across a majority of metal oxide material 14,but does not extend entirely across the metal oxide material. Thus, agap 62 remains between the filament and the metal oxide material 20. Ifthe metal oxide material 20 is electrically conductive material, the gap62 may be considered to define a programmable region of the metal oxidematerial 14 within the memory cell. If the metal oxide material 20 iselectrically insulative, then the gap between the filament andelectrically conductive structure would extend across metal oxide 20, aswell as extending across the shown portion of metal oxide 14 above thefilament.

The memory cells discussed above may be incorporated into electronicsystems. Such electronic systems may be used in, for example, memorymodules, device drivers, power modules, communication modems, processormodules, and application-specific modules, and may include multilayer,multichip modules. The electronic systems may be any of a broad range ofsystems, such as, for example, clocks, televisions, cell phones,personal computers, automobiles, industrial control systems, aircraft,etc.

The particular orientation of the various embodiments in the drawings isfor illustrative purposes only, and the embodiments may be rotatedrelative to the shown orientations in some applications. The descriptionprovided herein, and the claims that follow, pertain to any structuresthat have the described relationships between various features,regardless of whether the structures are in the particular orientationof the drawings, or are rotated relative to such orientation.

The cross-sectional views of the accompanying illustrations only showfeatures within the planes of the cross-sections, and do not showmaterials behind the planes of the cross-sections in order to simplifythe drawings.

When a structure is referred to above as being “on” or “against” anotherstructure, it can be directly on the other structure or interveningstructures may also be present. In contrast, when a structure isreferred to as being “directly on” or “directly against” anotherstructure, there are no intervening structures present. When a structureis referred to as being “connected” or “coupled” to another structure,it can be directly connected or coupled to the other structure, orintervening structures may be present. In contrast, when a structure isreferred to as being “directly connected” or “directly coupled” toanother structure, there are no intervening structures present.

Some embodiments include a memory cell. The memory cell has a firstelectrode material, and has a first metal oxide material over the firstelectrode material. The first metal oxide material has at least tworegions which differ in oxygen concentration relative to one another.One of the regions is a first region and another is a second region. Thefirst region is closer to the first electrode material than the secondregion, and has a greater oxygen concentration than the second region. Asecond metal oxide material is over and directly against the first metaloxide material. The second metal oxide material comprises a differentmetal than the first metal oxide material. A second electrode materialis over the second metal oxide material.

Some embodiments include a memory cell. The memory cell has a firstelectrode material, and has a first metal oxide material over the firstelectrode material. The first metal oxide material is selected from thegroup consisting of aluminum oxide, tantalum oxide, titanium oxide,nickel oxide, hafnium oxide and zirconium oxide. The first metal oxidematerial has at least two regions which differ in oxygen concentrationrelative to one another. One of the regions is a first region andanother is a second region. The first region is closer to the firstelectrode material than the second region, and has a greater oxygenconcentration than the second region. An electrically conductive secondmetal oxide material is over and directly against the first metal oxidematerial. A second electrode material is over and directly against thesecond metal oxide material.

Some embodiments include a method of forming a memory cell. A metaloxide material is formed over a first electrode material. An oxygen-sinkmaterial is formed over and directly against the metal oxide material. Asecond electrode material is formed over the oxygen-sink material. Themetal oxide material is treated to substantially irreversibly transferoxygen from a region of the metal oxide material to the oxygen-sinkmaterial and thereby subdivide the metal oxide material into at leasttwo regions. One of the regions nearest the oxygen-sink material isrelatively oxygen depleted relative to another of the regions.

In compliance with the statute, the subject matter disclosed herein hasbeen described in language more or less specific as to structural andmethodical features. It is to be understood, however, that the claimsare not limited to the specific features shown and described, since themeans herein disclosed comprise example embodiments. The claims are thusto be afforded full scope as literally worded, and to be appropriatelyinterpreted in accordance with the doctrine of equivalents.

We claim:
 1. A memory cell, comprising: first and second metal oxidesbetween first and second electrodes; the first and second electrodescomprising metal; the first metal oxide having at least two regionswhich differ in oxygen concentration relative to one another; one of theregions being a first region and another being a second region; thefirst region being closer to the first electrode than the second regionand having a greater oxygen concentration than the second region; thefirst metal oxide comprising one or both of hafnium oxide and zirconiumoxide; the second metal oxide being directly against the first metaloxide; the second metal oxide comprising a different metal than thefirst metal oxide; and wherein there is a substantially linearcontinuous oxygen-concentration gradient extending across an entirety ofthe first metal oxide.
 2. The memory cell of claim 1 wherein the firstmetal oxide consists essentially of hafnium oxide.
 3. The memory cell ofclaim 1 wherein the first metal oxide consists essentially of zirconiumoxide.
 4. The memory cell of claim 1 wherein the second metal oxideconsists essentially of ruthenium oxide, iridium oxide, nickel oxide,tantalum oxide or titanium oxide.
 5. The memory cell of claim 1 whereina thickness of the first metal oxide is within a range of from about 2nm to about 10 nm, and wherein a thickness of the second metal oxide isless than or equal to about 4 nm.
 6. A memory cell, comprising: firstand second metal oxides between first and second access/sense lines; thefirst metal oxide having at least two regions which differ in oxygenconcentration relative to one another; one of the regions being a firstregion and another being a second region; the first region being closerto the first access/sense line than the second region and having agreater oxygen concentration than the second region; the first metaloxide comprising one or both of hafnium oxide and zirconium oxide; thesecond metal oxide being directly against the first metal oxide; thesecond metal oxide comprising a different metal than the first metaloxide; and wherein there is a substantially linear continuousoxygen-concentration gradient extending across an entirety of the firstmetal oxide.
 7. A memory cell, comprising: a first electrode materialcomprising one or more of platinum, palladium, tantalum and titanium; afirst metal oxide material over the first electrode material; the firstmetal oxide material being selected from the group consisting ofaluminum oxide, tantalum oxide, titanium oxide, nickel oxide, hafniumoxide and zirconium oxide; the first metal oxide material having atleast two regions which differ in oxygen concentration relative to oneanother; one of the regions being a first region and another being asecond region; the first region being closer to the first electrodematerial than the second region, and having a greater oxygenconcentration than the second region; wherein there is a substantiallylinear continuous oxygen-concentration gradient extending across anentirety of the first metal oxide material; an electrically conductivesecond metal oxide material over and directly against the first metaloxide material; and a second electrode material over and directlyagainst the second metal oxide material.
 8. The memory cell of claim 7wherein the second metal oxide material comprises ruthenium oxide. 9.The memory cell of claim 7 wherein the second metal oxide materialconsists of ruthenium oxide.
 10. The memory cell of claim 7 wherein thefirst and second electrode materials comprise platinum or palladium. 11.The memory cell of claim 7 wherein the first and second electrodematerials comprise copper.